Ambient light calibration for energy efficiency in display systems

ABSTRACT

A method, system, and apparatus that can be used to operate a display device in an energy efficient manner. The energy efficient display device can effectively and efficiently compensate for changes in ambient light incident at a display screen of the display device using an internal ambient light sensor to provide control signals to a backlight driver.

BACKGROUND

1. Field of the Described Embodiments

The described embodiments relate generally to display devices. Inparticular, apparatus, method and system for providing an ambient lightcalibration factor used in a transmissive display are described.

2. Description of the Related Art

Solid state displays that use solid state elements such as liquidcrystal, or LC, for presenting visual content have become ubiquitous. Ina particular type of solid state display, a light source, referred to asa backlight, provides illumination that is used to form an image on aviewable display panel. For example, in those solid state displays thatutilize liquid crystal image elements (referred to as a liquid crystaldisplay, or LCD), the backlight can take the form of a discrete lightsource. In some cases, the backlight can take the form of a plurality oflight emitting diodes, or LEDs, that can provide a substantially whitelight. The white light, in turn, that can be projected through an imageforming layer having a plurality of image elements. The plurality ofimage elements can include a liquid crystal material that can beselectively rendered almost fully transparent to almost fully opaquebased upon an image signal applied to control elements. When combinedwith color filters (usually three color filters are used representingthe primary colors, red (R), blue (B), and green (G)), the plurality ofimage elements can form an array of pixels that can be used to create animage that can be viewed on a display panel that is typically covered bya protective layer formed of glass or plastic.

However, in order to provide a viewer with an acceptable (or in somecases, exceptional) viewing experience, the viewable image should appearbright and not washed out under all ambient light conditions. Forexample, in a viewing area that is brightly lit (naturally by sunlightor artificially using, for example, incandescent lighting), the imagepresented on the display panel can appear washed out due to the highambient light level reducing the overall contrast between the displayedimage and the surrounding area. Therefore, a number of displays attemptto maintain an acceptable viewing experience by using an ambient lightsensor to detect an ambient light level. The ambient light level is thenused to adjust the light output of the backlight. For example, theambient light sensor compensates for ambient light by making the displaybright enough for an acceptable viewing experience. Therefore, it isimportant for optimal viewing and power consumption that any change inambient light level detected by the ambient light sensor be effectivelycompensated by modifying the amount of light provided by the backlight.This is particularly true for energy efficient display systems since itis the backlight that consumes a substantial amount of the powerrequired to operate the display. Unfortunately, however, the opticalpath of a display system can include several optically active layersthrough with ambient light must pass before being detected by theambient light sensor. Each optically active layer can contribute to anoverall optical path tolerance, or variation. This variation can be onthe order of ±80% indicating that an ambient light level L₁ detected bythe ambient light sensor can only be correlated to an actual ambientlight level in the range of 0.2 L₁ to 1.8 L₁ making efficient backlightcontrol difficult. Moreover, this large variance can result in aconcomitantly large variance in display screen luminance.

In order to qualify as energy efficient (Energy Star, for example), aconsumer product, such as a display, must meet certain requirements forpower use and efficiency. Since the backlight typically accounts formost of the energy used by the display, it is important to be able toefficiently and effectively control the power used by the backlight inorder to meet a specific energy standard. Unfortunately, since theoptical path tolerance makes effective and efficient ambient lightcontrol of the backlight difficult to achieve, display manufacturerscompensate by reducing the overall light output of the backlight for allambient light conditions. This reduction in overall light outputtypically results in an inferior image presented by the display.

In view of the foregoing, there is a need for providing an energyefficient display that provides a viewer with a desirable viewingexperience under most if not all ambient light conditions.

SUMMARY OF THE EMBODIMENTS

A method for providing an ambient light calibration factor can beperformed by carrying out at least the following operations. Calibratinga light source to a target luminance value where the target luminancevalue corresponds to an ambient light condition, providing a calibratedlight by the light source, the calibrated light having a luminance valuewithin a range of the target luminance value, receiving the calibratedlight at a first part of an optical path at the target luminance level,the optical path having a plurality of elements each of which cause anassociated variance from the target luminance value of the lightprovided by the calibrated light source, detecting light received at thefirst part of the optical path by a light detector at a second part ofthe optical path at a second luminance level, the light detectorincluded in a display system, and calculating a calibration factor basedupon the relationship between the target luminance level and the secondluminance level, the calibration factor used by a system processor inthe display system to modify a control signal sent to a backlight driverunit, the control signal causing the backlight driver unit to output anamount of light in accordance with ambient light detected by the lightsensor.

A non-transitory computer readable medium for storing a computer programfor providing an ambient light calibration factor is described. Thecomputer program includes computer code for calibrating a light sourceto a target luminance value, the target luminance value corresponding toan ambient light condition, computer code for providing a calibratedlight by the light source, the calibrated light having a luminance valuewithin a range of the target luminance value, computer code forreceiving the calibrated light at a first part of an optical path at thetarget luminance level, the optical path having a plurality of elementseach of which cause an associated variance from the target luminancevalue of the light provided by the calibrated light source, computercode for detecting light received at the first part of the optical pathby a light detector at a second part of the optical path at a secondluminance level, and computer code for calculating a calibration factorbased upon the relationship between the target luminance level and thesecond luminance level.

A system includes a calibrated light source arranged to providecalibrated light having a luminance value within a range of the targetluminance values, the target luminance value corresponding to an ambientlight condition, a light detector arranged to detect the calibratedlight received at a first part of an optical path at the targetluminance level, the optical path having a plurality of elements each ofwhich cause an associated variance from the target luminance value,wherein the detected light is at a second luminance level, and aprocessor coupled to the light detector for calculating a calibrationfactor based upon the relationship between the target luminance leveland the second luminance level.

Other apparatuses, methods, features and advantages of the describedembodiments will be or will become apparent to one with skill in the artupon examination of the following figures and detailed description. Itis intended that all such additional apparatuses, methods, features andadvantages be included within this description be within the scope ofand protected by the accompanying claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments will be readily understood by the following detaileddescription in conjunction with the accompanying drawings, wherein likereference numerals designate like structural elements, and in which:

FIG. 1 graphically illustrates the data presented in Table 1 showingrepresentative Lambertian angular response curve and representativenon-Lambertian angular response curve typical of a less costly lightsensor.

FIG. 2 shows representative display undergoing calibration wherecalibration system

FIG. 3 shows representative calibration system in accordance with thedescribed embodiments.

FIG. 4 shows a calibration factor CF stored in a display device inaccordance with the described embodiments.

FIG. 5 shows a flowchart detailing a process for generating an ambientlight calibration factor in accordance with the described embodiments.

FIG. 6 shows a flowchart describing a process for storing an ambientlight calibration factor CF in accordance with the describedembodiments.

FIG. 7 shows a flowchart describing a process for utilizing an ambientlight calibration factor CF in a display system in accordance with thedescribed embodiments.

FIG. 8 shows a flowchart describing a process for validating acalibration coefficient in accordance with an embodiment of theinvention.

FIG. 9 is an exploded perspective view of liquid crystal display (LCD)in accordance with an embodiment of the invention.

FIG. 10 is a cross-sectional view showing one side of the LCD shown inFIG. 9 in an assembly state.

DESCRIBED EMBODIMENTS

In the following paper, numerous specific details are set forth toprovide a thorough understanding of the concepts underlying thedescribed embodiments. It will be apparent, however, to one skilled inthe art that the described embodiments may be practiced without some orall of these specific details. In other instances, well known processsteps have not been described in detail in order to avoid unnecessarilyobscuring the underlying concepts.

This paper discusses a method, system, and apparatus that can be used tooperate a display device in an energy efficient manner. In oneembodiment, the energy efficient display device can effectively andefficiently compensate for changes in ambient light incident at adisplay screen of the display device using an internal ambient lightsensor to provide control signals to a backlight driver. The internalambient light sensor can be part of a stack of optical elements includedin an optical path through which the ambient light must pass in order tobe detected. In the described embodiment, the optical elements caninclude a protective display layer, and a plurality of apertures and/oropenings in either or both the protective display layer and a maskinglayer (such as ink). The optical elements can also include material usedform a light pipe and any angular variations in the light pipe used fordirecting the ambient light to the ambient light detector. Variationscan also be caused by the ambient light detector itself. For example,variations due to light sensor material as well as angular variationsdue to mechanical tolerances of the display can all add to the overalloptical tolerance. As described above, for conventional display systems,the optical path associated with the ambient light detector can have anoverall optical tolerance on the order to about ±80%. However, in thedescribed embodiments, the overall optical tolerance of the ambientlight detector optical stack can be reduced to about ±5% using at leasta calibration factor (CF) to modify a signal used to control a backlightdriver unit. In some embodiments, an alignment factor (AF) can also beused to modify the backlight driver unit control signal.

In the described embodiments, the calibration factor (CF) can compensatefor the overall luminance variation caused by the elements in theoptical path that the ambient light must follow in order to reach theambient light detector. In this way, the correlation between theluminance value of the ambient light detected at the ambient lightsensor and the actual luminance value is greatly improved. Using theexample above, with the overall optical stack tolerance reduced to ±5%,the ambient light level detected by an internal ambient light sensor canbe correlated to the actual ambient light level in the range of 0.95 L₁to 1.05 L₁ which is a substantial improvement over the prior art rangeof 0.2 L₁ to 1.8 L₁.

An ideal light sensor will exhibit what is referred to as a Lambertianangular response in which the output of the light sensor is proportionalto the cosine of the angle of incidence where an angle of incidence ofabout zero (0°) degrees is normal to the display screen. However, lesscostly light sensors typically utilize photo-detectors that do notexhibit the Lambertian response. On the contrary, the typical angularresponse of the less costly light sensors is generally not wellcorrelated to a cosine curve and is typically determined experimentallyas shown in Table 1 enumerating and contrasting a Lambertian angularresponse and a non-Lambertian angular response.

TABLE 1 Lambertian Non-Lambertian Angle Response Response −90 0.00 0.00−80 0.17 0.00 −70 0.34 0.02 −60 0.50 0.10 −50 0.64 0.25 −40 0.77 0.43−30 0.87 0.65 −20 0.94 0.83 −10 0.98 0.95 0 1.00 1.00 10 0.98 0.95 200.94 0.83 30 0.87 0.65 40 0.77 0.43 50 0.64 0.25 60 0.50 0.10 70 0.340.02 80 0.17 0.00 90 0.00 0.00

FIG. 1 graphically illustrates the data presented in Table 1 showingrepresentative Lambertian angular response curve 102 and representativenon-Lambertian angular response curve 104 typical of a less costly lightsensor. During calibration and characterization of the display device,external ambient light sensors that exhibit a Lambertian (or essentiallyLambertian) angular response can be used to detect an ambient lightlevel. For example, FIG. 2 shows representative display 200 undergoingcalibration where calibration system ambient light sensor 202 having aLambertian response can be oriented to have an angle of incidence ofabout 90° relative to normal N of display screen 204. In thisorientation, sensor 202 can capture an optimal amount of diffuse ambientlight provided by light sources 206. However, display system ambientlight sensor 208 is one that generally is not expected to exhibit theLambertian angular response curve 102 but more likely to have an angularresponse more like that of non-Lambertian angular response curve 104. Inthe described embodiment, angular calibration factor AF can be used toaccount for the differences in angular response between the calibrationdata provided by calibration system ambient light sensor 202 and displaysystem light sensor 208. In this way, angular calibration factor AF canbe used to modify the operation of the backlight driver unit separatelyor in combination with calibration factor CF.

These and other embodiments are discussed below with reference to FIGS.1-10. However, those skilled in the art will readily appreciate that thedetailed description given herein with respect to these figures is forexplanatory purposes only and should not be construed as limiting.

FIG. 3 shows representative calibration system 300 in accordance withthe described embodiments. Calibration system 300 can be used todetermine validated calibration factors (CF) that can be used to modifya control signal. The control signal can be used to control an amount oflight output from an illumination source such as a backlight. Themodification of light provided by the backlight can be in accordancewith a change in an ambient light level detected by an ambient lightdetector. Calibration system 300 can be used in a laboratory environmentor in a manufacturing environment to accurately determine consistent andvalidated calibration factors for a particular display system under avariety of ambient light conditions. Calibration system 300 can includeat least system under test (SUT) 302, light source 304 and light sensor306 electrically connected to and part of light meter 308. SUT 302 cantake the form of a solid state display along the lines of a liquidcrystal display, or LCD. Light source 304 can take the form of anincandescent light, a CCFL or a plurality of light emitting diodes, orLEDs. Light sensor 306 can include a photo detector unit and associatedcircuitry. Enclosure 310 can optically isolate SUT 302, light source 304and light sensor 306 from the external environment. In this way, thecalibration process can be unaffected by any extraneous light notoriginating from light source 304. Enclosure 310 can take the form of ashroud formed of opaque material such as black cloth or otherappropriate materials.

Light meter 308 can receive electrical signals from light sensor 306indicative of an amount of light detected by a photo-detector includedin light sensor 306. In the described embodiment, light sensor 306 canbe placed in close proximity to SUT 302 in order to accurately simulatethe amount and intensity of light from light source 304 that reaches SUT302. By placing light sensor 306 in close proximity to SUT 302, anyattenuation of light from light source 304 can be taken into accountproviding a more accurate calibration of light source 304 and ultimatelycalibration factor CF for SUT 302. For example, when light source 304provides light having luminance level L_(source), then any attenuationcan result in light received at SUT 302 having a reduced luminance valueL_(SUT) that is less than L_(source). Light sensor 306 can be placed inclose proximity to SUT 302 having luminance value L_(sense) that isessentially the same as that of the light received at SUT 302, namelyL_(sense) is proportional to L_(SUT).

Light meter 308 can be electrically connected to process computer 312.Process computer 312 can be a standalone unit or be incorporated into aseparate calibration unit either of which can be coupled directly to adata port of SUT 302. In any case, process computer 312 can providecontrol signals to programmable power supply 314 in response to inputsignal 316 received from light meter 308. Input signal 316 can, in turn,be directly related to the luminance L_(sense) of light from lightsource 304 received at light sensor 306. In this way, control loop 318can be used by process computer 312 to calibrate light source 304. Inone embodiment, light source 304 can be calibrated to simulate a user'sexpected ambient light level at SUT 302. For example, light source 304can be calibrated to provide an ambient light level having a luminancevalue of about 300 lux (lx) where 1 lx is equal to 1 lumen (lm) persquare meter (m²).

In one embodiment, control loop 318 can operate as follows. Based upon atarget luminance value provided to process computer 312, processcomputer 312 can provide control signal 320 to programmable power supply314. Programmable power supply 314 can respond to control signal 320 bysending power signal 322 to light source 304. Power signal 322 can causelight source 304 to either increase or decrease an amount of lightdetected at light sensor 306. Light sensor 306, in turn, generate signal324 that can be passed to light meter 308. Light meter 308 can passsignal 316 indicative of the amount of light from light source 304detected at light sensor 306. Process computer 312 can evaluateinformation provided by signal 316 in order to determine if light source304 is providing light within an acceptable range of a target luminancevalue. Based upon the evaluation, process computer 312 determines thatlight source 304 is providing light within the acceptable range of thetarget luminance value, then the control loop ends, otherwise, processcomputer 312 updates control signal 320 in accordance with theevaluation of the light output of light source 304.

SUT 302 can include internal light sensor 326. Light from light source304 reaching SUT 302 as calibrated ambient light L_(SUT) can reachinternal light sensor 326 by following optical path 328. As describedabove, optical path 328 can present a number of elements each of whichcan affect the detection of ambient light L_(SUT) by internal lightsensor 326. Since light source 304 has been calibrated to provide lightin the acceptable range of the target luminance value, the luminance ofambient light L_(SUT) can be provided to SUT 302 by process computer 312as a corrected light meter reading (LC≈L_(SUT)). In this way, the lightlevel (LS) detected by internal sensor 326 can be used to determinecalibration factor CF according to equation (1):

$\begin{matrix}{{CF} = \frac{LC}{LS}} & {{Eq}\mspace{14mu}(1)}\end{matrix}$

In order to validate calibration factor CF, SUT 302 can reportcalibration factor CF to process computer 312 for validation. Byvalidating calibration factor CF, process computer 312 can verify thatcalibration factor CF is within an allowable range of calibrationfactors. This allowable range of calibration factors can be based upon,for example, tolerances of the various optical elements included in theoptical path. Such elements can include, for example, light pipes, lightsensor angle, the light sensor, and so on as described above.

In the described embodiment, process computer 312 can validatecalibration factor CF as follows. Process computer 312 can determinepower level P provided by, power source 330 by reading power meter 332at, for example, a user's typical ambient light level L_(typical) asdetected by screen luminance meter 334. Power level P can then becompared to design limits based upon energy standards (such as thoseprovided by the Environmental Protection Agency, or EPA, as determinedby the EnergyStar standard) and any power consumption tolerance of SUT302. In some cases, process computer 312 can also verify that lightemitted by the display of SUT 302 is within established design limits.

As part of the validation of the calibration factor, process computer312 can determine power level P_(L) corresponding to a condition of lowambient light level and power level P_(H) corresponding to a conditionof high ambient light level. Process computer 312 use the determinedvalues of P_(L) and P_(H) to calculate average weighted power Pavg basedupon equation (2)Pavg=WH×PH+WL×PL  Eq (2)

-   -   where:        -   Pavg is weighted average power;        -   WH is brighter (higher) lighting condition weight factor;        -   PH is brighter (higher) lighting condition power level;        -   WL is darker (lower) lighting condition weight factor; and        -   PL is darker (lower) lighting condition power level.

In the described embodiment, weighting factor WH is typically greaterthan weighting factor WL in order to provide a more conservative (powerwise) estimate of the power consumption of SUT 302. For example,weighting factor WH can be on the order of 0.8 whereas weighting factorWL can be on the order of 0.2.

As further shown in FIG. 4 calibration factor CF can be stored in SUT302 in the form of display device 400. Display device 400 can includelight sensor 402, system processor 404, and memory device 406 that cantake the form of non-volatile memory such as EEPROM. Display device 400can also include backlight driver 408 configured to provide controlsignals to a backlight unit (not shown) that provides illumination usedto provide a displayable image on a display panel. Calibration factor CFcan be stored in display system 400 in one embodiment as follows.Process computer 312 can be connected to system 400 by way of aninput/output data port such as a USB data port. Process computer 312 cancause display device 400 to enter a calibration mode by process computer312 sending trigger signal 410 to system processor 404.

In one embodiment, trigger signal 410 can include information such ascorrected light meter reading LC. In calibration mode, system processor404 can sample light sensor 402 for an indication a luminance value oflight received through optical path 412 corresponding to ambient light414 provided by light source 304. System processor 404 can thencalculate calibration factor CF based upon the sampled light reading LSand light meter reading LC according to equation (1). Once calculated,calibration factor CF can be stored in memory device 406. Oncecalibration factor CF is stored in memory device 406, system processor404 can cause display device 400 to exit the calibration mode. In oneembodiment, display device 400 exits the calibration mode after systemprocessor 404 has reported calibration factor CF to process computer312.

Once calibration factor CF has been stored in memory device 406 anddisplay device 400 is no longer in calibration mode, system processor404 can retrieve calibration factor CF from memory device 406 as well asany user settings 416 (such as a most recent brightness) from memorydevice 406. During normal operation of display device 400, systemprocessor 404 can sample light received at light sensor 402 anddetermine calibrated ambient light level LA as equation 3:LA=CF×LS  eq. (3)

System 400 can apply calibrated ambient light level LA and any usersettings to ambient light control function 418 executed by systemprocessor 404. Ambient light control function 418 can issue command 420to backlight driver 408 that can respond by, for example, changing abacklight duty cycle and/or a backlight phase.

FIG. 5 shows a flowchart detailing process 500 for generating acalibration factor for modifying a control signal used by a backlightdriver to compensate for an ambient light condition in accordance withthe described embodiments. Process 500 can begin at 502 by calibrating alight source. The light source can be calibrated to a target luminancevalue. The target luminance value can correspond to an expected ambientlight condition experienced by a display device. Next at 504, acalibration factor CF is determined based upon, in part, the lightprovided by the calibrated light source. An ambient light sensorinternal to a display device detects the light provided by thecalibrated light source having a known target luminance. The luminancevalue of the light detected by the internal ambient light sensor is thencompared to the light provided by the calibrated light source at thetarget luminance. The calibration factor CF is that ratio of thedetected luminance value and the target luminance value. The calibrationfactor CF can be used to compensate for variations caused by elements inan optical path that the light from the light source must travel toreach the internal ambient light detector. Next, at 506, the calibrationfactor CF is validated. By validation, it is meant that the energy usageand screen luminance values are evaluated for compliance to both systemdesign standard and energy efficiency standard.

FIG. 6 shows a flowchart describing process 600 for storing an ambientlight calibration factor CF in accordance with the describedembodiments. Process 600 can begin at 602 by triggering a systemprocessor to enter a calibration mode. In the calibration mode, thesystem processor can receive data from an internal light sensor at 604.The data received from the internal light sensor can correspond toambient light provided by a calibrated light source. At 606, the systemprocessor can then calculate a calibration factor CF based upon the datareceived from the internal light sensor and data received from anexternal circuit such as a process computer. The data received from theprocess computer can include a corrected light meter reading. At 608,the calibration factor CF can be stored in a memory device and reportedto the process computer at 610 at which point, the system processor canexit the calibration mode at 612.

FIG. 7 shows a flowchart describing process 700 for utilizing an ambientlight calibration factor CF in a display system in accordance with thedescribed embodiments. Process 700 can begin at 702 by the systemcontroller retrieving the calibration factor CF from the memory device.At 704, during normal operation, the system processor can receive datafrom the internal ambient light sensor. At 706, a calibrated ambientlight level is determined based upon the calibration factor CF and thedata received from the internal sensor. At 708, the calibrated ambientlight level and any user settings are retrieved from the memory device.They are applied to an ambient light brightness control function at 710.In one embodiment, the ambient light brightness control function can beexecuted by the system processor. At 712, the ambient light brightnesscontrol function can modify the output of a backlight driver. In oneembodiment, a duty cycle and phase of backlight driver can be modified.

FIG. 8 shows a flowchart detailing process 800 for validating acalibration coefficient in accordance with the described embodiments.Process 800 can being at 802 by determining a power level P provided toa display system by a power source at a user's typical ambient lightlevel. Next at 804, the power level P is compared to design limit powerlevels based in part upon an energy standard. For example, the energystandard includes power limits defining what is considered to be anenergy efficient display. At 806, if the power level P does not meet thestandard, then at 808 the calibration factor is re-calculated andcontrol is passed back to 802. On the other hand, if the power leveldoes meet the standard, then at 810 a determination of an amount oflight emitted by the display is determined. At 812, the amount of lightemitted by the display is then compared to design limits for thedisplay. If the light emitted by the display does not meet the designlimits, then at 808, the calibration factor is recalculated and controlis passed back to 802, otherwise, the calibration coefficient isacceptable at 814.

FIG. 9 is an exploded perspective view of liquid crystal display (LCD)900 in accordance with an embodiment of the invention. FIG. 10 is across-sectional view showing one side of LCD 900 shown in FIG. 9 in anassembly state. Referring to FIGS. 9 and 10, LCD 900 includes supportmain 914, backlight unit 950, and liquid crystal display panel 906stacked within the support main 914, and top casing 902 for surroundingthe edges of liquid crystal display (LCD) panel 906 and lateral portionsof the support main 914. LCD panel 906 includes liquid crystalintervened between front substrate 905 and rear substrate 903, andspacers for maintaining a gap between the front substrate 905 and rearsubstrate 903. A color filter and a black matrix are formed in the frontsubstrate 905 of the LCD panel 906. Signal lines, such as data lines andgate lines, are formed in the rear substrate 903 of LCD panel 906. Athin film transistor (hereinafter referred to as a “TFT”) is formed atcrossings of the data lines and the gate lines. The TFT switches a datasignal to transmit the data signal from the data line to a liquidcrystal cell, in response to a scan signal gate pulse transmitted fromthe gate line. A pixel electrode is formed in a pixel area between thedata line and the gate line. Further, pad regions to which the datalines and the gate lines are respectively coupled are formed in one sideof rear substrate 903. A driver integrated circuit (not shown) forapplying a driving signal to the TFT is mounted is attached to each padregion. The data signal, transmitted from the driver integrated circuit,is sent to the data lines and also supplies a scan signal to the gatelines. An upper polarization sheet is attached to front substrate 905 ofLCD panel 906, and a lower polarization sheet is attached to rearsubstrate 903 of rear substrate 903.

Backlight unit 950 includes plurality of light sources for providinglight to LCD panel 906. The light sources can be LED devices. The dutyratio of the output signal of the inverter isT_(on)×100/(T_(on)+T_(off)), where ‘T_(on)’ denotes a turn-on period ofthe light source and ‘T_(off)’ denotes a turn-off period of the lightsource. The duty ratio of the output signal determines the luminance ofthe light source.

The plurality of optical sheets 908 stacked over the diffusion sheet 910redirects light incident from the diffusion sheet 910 to be incidentperpendicular to the liquid crystal display panel 906, thus improvingoptical efficiency. To this end, the optical sheets 908 include twosheets of prism sheets and two sheets of spreading sheets. The twosheets of prism sheets stand a travel angle of spreading light, emittedfrom the diffusion sheet 910, in a direction vertical to the liquidcrystal display panel 906. The two sheets of spreading sheets spread thevertically incident light again. The top casing 902 is formed in arectangular belt having a plan portion and a lateral portion, which arecurved at a right angle to each other and surrounds the corners of theLCD panel 906 and the sides of the support main 914.

The various aspects, embodiments, implementations or features of thedescribed embodiments can be used separately or in any combination.Various aspects of the described embodiments can be implemented bysoftware, hardware or a combination of hardware and software. Thedescribed embodiments can also be embodied as computer readable code ona computer readable medium for controlling manufacturing operations oras computer readable code on a computer readable medium for controllinga manufacturing line. The computer readable medium is any data storagedevice that can store data which can thereafter be read by a computersystem. Examples of the computer readable medium include read-onlymemory, random-access memory, CD-ROMs, DVDs, magnetic tape, and opticaldata storage devices. The computer readable medium can also bedistributed over network-coupled computer systems so that the computerreadable code is stored and executed in a distributed fashion.

While the embodiments have been described in terms of several particularembodiments, there are alterations, permutations, and equivalents, whichfall within the scope of these general concepts. It should also be notedthat there are many alternative ways of implementing the methods andapparatuses of the present embodiments. For example, although anextrusion process is preferred method of manufacturing the integraltube, it should be noted that this is not a limitation and that othermanufacturing methods can be used (e.g., injection molding). It istherefore intended that the following appended claims be interpreted asincluding all such alterations, permutations, and equivalents as fallwithin the true spirit and scope of the described embodiments.

1. A method, comprising: calibrating a light source to a targetluminance value, the target luminance value corresponding to an ambientlight condition; providing a calibrated light by the light source, thecalibrated light having a luminance value within a range of the targetluminance value; receiving the calibrated light at a first part of anoptical path at the target luminance level, the optical path having aplurality of elements each of which cause an associated variance fromthe target luminance value of the light provided by the calibrated lightsource; detecting light received at the first part of the optical pathby a light detector at a second part of the optical path at a secondluminance level, the light detector included in a display system; andcalculating a calibration factor based upon the relationship between thetarget luminance level and the second luminance level, the calibrationfactor used by a system processor in the display system to modify acontrol signal sent to a backlight driver unit, the display signalcausing the backlight driver unit to output an amount of light inaccordance with ambient light detected by the light sensor.
 2. Themethod as recited in claim 1, wherein the calibrating the light sourcecomprises: receiving light from the light source at a light sensor, thelight sensor and the light source being optically isolated from externalenvironment; sending an indication of the luminance of the lightreceived at the light sensor to a light meter connected to the lightsource; generating a luminance value by the light meter based upon theindication provided by the light sensor.
 3. The method as recited inclaim 2, comprising: forwarding the luminance value by the light meterto a process computer; and comparing the luminance value to the targetluminance value.
 4. The method as recited in claim 3, comprising: whenthe luminance value is not within the range of the target luminancevalue based upon the comparing, forwarding a corrective command to aprogrammable power supply.
 5. The method as recited in claim 4,comprising: modifying a power level of the light source by theprogrammable power supply based upon the corrective command.
 6. Themethod as recited in claim 1, further comprising: storing thecalibration factor in the display system by the system processor.
 7. Themethod as recited in claim 6, further comprising: setting the displaysystem to calibration mode; receiving data by the system processor fromthe light sensor in accordance with light detected by the light sensor;calculating the calibration factor; storing the calibration factor in amemory device; reporting the calibration factor to an external processcomputer; and exiting the calibration mode.
 8. The method as recited inclaim 7, further comprising: when the display system is in normaloperating mode, the system processor, retrieves the calibration factorfrom the memory device; receives data from the light sensor, the data inaccordance with an ambient light level; determines a calibrated ambientlight level; retrieves user settings from the memory device; applies thecalibrated ambient light level and the user settings to an ambientbrightness control function that modifies a duty cycle and phase of abacklight driver in accordance with the ambient light level.
 9. Themethod as recited in claim 1, further comprising: validating thecalibration factor comprising: determining a power level P provided tothe display system by a power source at a user's typical ambient lightlevel; comparing the power level P to design limits based upon an energystandard; determining an amount of light emitted by the display system;and verifying that the amount of light emitted by the display is withinestablished design limits.
 10. The method as recited in claim 9, furthercomprising: wherein the power level P is an average power level Pavg,the average power level being a sum of weighted power levels, theweighted power levels including at least a brighter lighting conditionpower level and a darker lighting condition power level.
 11. Anon-transitory computer readable medium for storing a computer programfor providing an ambient light calibration factor, comprising: computercode for calibrating a light source to a target luminance value, thetarget luminance value corresponding to an ambient light condition;computer code for providing a calibrated light by the light source, thecalibrated light having a luminance value within a range of the targetluminance value; computer code for receiving the calibrated light at afirst part of an optical path at the target luminance level, the opticalpath having a plurality of elements each of which cause an associatedvariance from the target luminance value of the light provided by thecalibrated light source; computer code for detecting light received atthe first part of the optical path by a light detector at a second partof the optical path at a second luminance level; and computer code forcalculating a calibration factor based upon the relationship between thetarget luminance level and the second luminance level.
 12. The computerreadable medium as recited in claim 11, wherein the calibrating thelight source comprises: computer code for receiving light from the lightsource at a light sensor, the light sensor and the light source beingoptically isolated from external environment; computer code for sendingan indication of the luminance of the light received at the light sensorto a light meter connected to the light source; computer code forgenerating a luminance value by the light meter based upon theindication provided by the light sensor.
 13. The computer readablemedium as recited in claim 12, comprising: computer code for forwardingthe luminance value by the light meter to a process computer; andcomputer code for comparing the luminance value to the target luminancevalue.
 14. The computer readable medium as recited in claim 13,comprising: computer code for forwarding a corrective command to aprogrammable power supply when the luminance value is not within therange of the target luminance value based upon the comparing.
 15. Thecomputer readable medium as recited in claim 14, comprising: computercode for modifying a power level of the light source by the programmablepower supply based upon the corrective command.
 16. The computerreadable medium as recited in claim 11, further comprising: computercode for storing the calibration factor in the display system by thesystem processor.
 17. The computer readable medium as recited in claim16, further comprising: computer code for setting the display system tocalibration mode; computer code for receiving data by the systemprocessor from the light sensor in accordance with light detected by thelight sensor; computer code for calculating the calibration factor;computer code for storing the calibration factor in a memory device;computer code for reporting the calibration factor to an externalprocess computer; and computer code for exiting the calibration mode.18. The computer readable medium as recited in claim 17, furthercomprising: when the display system is in normal operating mode, thesystem processor, retrieves the calibration factor from the memorydevice; receives data from the light sensor, the data in accordance withan ambient light level; determines a calibrated ambient light level;retrieves user settings from the memory device; applies the calibratedambient light level and the user settings to an ambient brightnesscontrol function that modifies a duty cycle and phase of a backlightdriver in accordance with the ambient light level.
 19. The computerreadable medium as recited in claim 11, further comprising: validatingthe calibration factor comprising: computer code for determining a powerlevel P provided to the display system by a power source at a user'stypical ambient light level; computer code for comparing the power levelP to design limits based upon an energy standard; computer code fordetermining an amount of light emitted by the display system; andcomputer code for verifying that the amount of light emitted by thedisplay is within established design limits.
 20. The computer readablemedium as recited in claim 11, further comprising: wherein the powerlevel P is an average power level Pavg, the average power level being asum of weighted power levels, the weighted power levels including atleast a brighter lighting condition power level and a darker lightingcondition power level.
 21. A system, comprising: a calibrated lightsource arranged to provide calibrated light having a luminance valuewithin a range of the target luminance values, the target luminancevalue corresponding to an ambient light condition; a light detectorarranged to detect the calibrated light received at a first part of anoptical path at the target luminance level, the optical path having aplurality of elements each of which cause an associated variance fromthe target luminance value, wherein the detected light is at a secondluminance level; and a processor coupled to the light detector forcalculating a calibration factor based upon the relationship between thetarget luminance level and the second luminance level.
 22. The system asrecited in claim 21, the light source is calibrated by receiving lightfrom the light source at a light sensor, the light sensor and the lightsource being optically isolated from external environment, sending anindication of the luminance of the light received at the light sensor toa light meter connected to the light source, and generating a luminancevalue by the light meter based upon the indication provided by the lightsensor, forwarding the luminance value by the light meter to a processcomputer, comparing the luminance value to the target luminance value.23. The system as recited in claim 22, wherein when the luminance valueis not within the range of the target luminance value based upon thecomparing, the process computer forwards a corrective command to aprogrammable power supply.
 24. The system as recited in claim 23,comprising: wherein the programmable power supply modifies a power levelof the light source by the programmable power supply based upon thecorrective command.
 25. An apparatus, comprising: means for calibratinga light source to a target luminance value, the target luminance valuecorresponding to an ambient light condition; means for providing acalibrated light by the light source, the calibrated light having aluminance value within a range of the target luminance value; means forreceiving the calibrated light at a first part of an optical path at thetarget luminance level, the optical path having a plurality of elementseach of which cause an associated variance from the target luminancevalue of the light provided by the calibrated light source; means fordetecting light received at the first part of the optical path by alight detector at a second part of the optical path at a secondluminance level, the light detector included in a display system; andmeans for calculating a calibration factor based upon the relationshipbetween the target luminance level and the second luminance level, thecalibration factor used by a system processor in the display system tomodify a control signal sent to a backlight driver unit, the displaysignal causing the backlight driver unit to output an amount of light inaccordance with ambient light detected by the light sensor.
 26. Theapparatus as recited in claim 25, wherein the calibrating the lightsource comprises: means for receiving light from the light source at alight sensor, the light sensor and the light source being opticallyisolated from external environment; means for sending an indication ofthe luminance of the light received at the light sensor to a light meterconnected to the light source; means for generating a luminance value bythe light meter based upon the indication provided by the light sensor.